In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Tools of the above-related kind are used to ream holes in metallic workpieces by chip removing or cutting machining with the purpose of giving the holes a cylindrical, smooth surface having an accurately determined diameter. A product, which advantageously can be machined by means of the tool, is tube blanks, the interior of which for different reasons has to be dimensionally accurate and have a high surface smoothness. A usual method for, in practice, carrying out machining of just tube blanks is so-called pull boring. The tool is then connected with one end of a drawbar, which has an outer diameter that is smaller than the inner diameter of the tube blank, and which in a first step is brought through the tube blank so that the tool can be applied on the free end thereof, e.g. via a threaded joint, after which the reaming operation is undertaken by a combination of rectilinear and rotary motions between the tool and the tube blank. Usually, the tool is fed longitudinally by being pulled through the interior of the tube blank without rotating, at the same time as the tube blank is brought to rotate. By these relative motions, the cutting inserts detachably mounted on the tool head will, in a way characteristic of cutting machining, remove chips from the hole wall while generating a cylinder surface having good dimensional accuracy and high surface smoothness.
The prismatic cutting inserts, which have been used in previously known reaming tools of the kind in question, are formed with two opposite chip surfaces; an upper side and a longer under side; two opposite end surfaces; as well as two clearance surfaces that extend from the upper side toward the end surfaces and lean downward toward the same. Therefore, along each chip surface, there is only one cutting edge usable for one and the same tool, viz. in a transition toward an individual clearance surface. This means that the cutting insert includes in total only two alternately usable cutting edges. Because the volume of the cutting insert, and thereby the quantity of costly material (cemented carbide), thus becomes disproportionately great in relation to the number of usable cutting edges, the operating economy for the user per cutting insert will be fairly mediocre. Accordingly, the known cutting insert is only invertible, but not indexable so as to allow utilization of further edges of the cutting insert as cutting edges. Another disadvantage of the known cutting inserts is that the same tend to be displaced in the appurtenant seat or even come loose from the same in case the same are subjected to a reverse interaction of forces. Although the present reaming method usually is reliable and well functioning, accordingly, every now and then mishaps occur requiring that the machining operation is interrupted and that the tool is retracted out of the hole. Examples of mishaps are that one or more cutting inserts become damaged or come loose from the tool head, or that the driving machine facility stops. In connection with such retractions, the intact cutting inserts of the tool head may come to be jammed against the interior of the tube blank, wherein the cutting inserts are subjected to a reversed interaction of forces that tends to dislodge the same out of the appurtenant seats. Instead of being held pressed automatically against all support surfaces in the seat by the cutting forces during operation, the cutting insert is subjected to an aim to be distanced from above all the axial support surface, but also the tangential support surface. If a screw is used as clamping member, this has, however, poor chances of withstanding the reversed forces. Characteristic of a screw is that it has good tensile strength, but considerably inferior bending strength. Therefore, if the tool has a moderate diameter and the cutting inserts as well as the screws are small, it easily occurs that the screws yield to the unpredicted, reversed forces that are applied to the cutting insert.